1. Field of the Invention
The present invention relates to a secondary battery power control method for an interconnected system that supplies power to a power system, and includes a power generator (e.g., wind power generator) that changes in output power, and an electric power storage-compensation device that includes a sodium-sulfur battery.
2. Description of Related Art
In recent years, a natural-energy power generator that generates power by utilizing wind power, solar power, geothermal energy, or the like has attracted attention, and has been put to practical use. A natural-energy power generator is a clean power generator that utilizes an inexhaustible natural energy source instead of limited resources (e.g., petroleum), and can suppress carbon dioxide emissions. Therefore, various companies, autonomous bodies, and the like have increasingly employed a natural-energy power generator.
However, since the amount of natural energy obtained varies from hour to hour, a natural-energy power generator inevitably changes in output. This is an obstacle to widespread use of a natural-energy power generator. Therefore, when employing a natural-energy power generator, it is preferable to construct an interconnected (power generation) system by combining the natural-energy power generator with an electric power storage-compensation device that mainly includes a plurality of secondary batteries for preventing the obstacle.
In particular, a sodium-sulfur battery out of secondary batteries has a high energy density, achieves a high output within a short time, and exhibits a rapid response. Therefore, a sodium-sulfur battery may suitably be used to compensate for a change in output of a natural-energy power generator that may occur of the order of several hundred milliseconds to several seconds by utilizing a bidirectional converter that controls charging and discharging in combination with the sodium-sulfur battery. In other words, an interconnected system that includes a natural-energy power generator and an electric power storage-compensation device that includes a plurality of sodium-sulfur batteries is a desirable power generation system.
FIG. 3 is a graph showing a case of a change in power generated by a wind power generator and a planned value with respect to time when using an interconnected system that includes a wind power generator and an electric power storage-compensation device. As shown in FIG. 3, power generated by the wind power generator is stored in the power storage-compensation device in a time zone (1) (e.g., nighttime) in which power is not supplied to the power system. On the other hand, in a time zone (2) (e.g., daytime) power generated by the wind power generator is supplied to the power system, and power required to reach the planned value is discharged from the power storage-compensation device, and supplied to the power system. When using the interconnected system that includes the wind power generator and the power storage-compensation device, power output from the wind power generator changes every moment, as shown in FIG. 3. Therefore, the charging/discharging pattern of a secondary battery included in the power storage-compensation device also changes in order to absorb a change in output power.
No related art that has the same object as that of the present invention described later has been found. Patent Documents 1 and 2 disclose related technology, for example.